Method for treating carbon dioxide-containing waste gas

ABSTRACT

A hydrocarbon-containing gas is guided to waste gas containing carbon dioxide and the carbon dioxide is at least partially converted into carbon monoxide and hydrogen when reacted with the hydrocarbon. The waste gas is used with the carbon monoxide hydrogen mixture for an additional combustion process.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national stage of International Application No. PCT/EP2012/050015, filed Jan. 12, 2012 and claims the benefit thereof. The International Application claims the benefit of German Application No. 10 2011 002 612.6 filed on Jan. 13, 2011, both applications are incorporated by reference herein in their entirety.

BACKGROUND

Described below is a method for treating a carbon dioxide-containing waste gas. Hot waste gases containing a high percentage of carbon dioxide (CO₂) are frequently produced in industrial processes, in particular in steelmaking. This happens for example during the operation of what is known as a converter (BOF=Blast Oxygen Furnace). During such a process, hot waste gases attaining a temperature of approx. 1700° C. are released. Some of the waste heat is used in a steam generator. The remainder is cooled down by an evaporation cooler. Dust particles are then removed from the waste gases by filtering. In order to pass through the filter system the waste gas must not exceed a temperature of more than 180° C.

The accumulating waste gas, in particular in converters, contains a large percentage of carbon dioxide (CO₂) in many process sections. If this carbon dioxide escapes into the environment, it contributes toward the so-called greenhouse effect.

SUMMARY

The method for treating a carbon dioxide-containing waste gas reduces the percentage of the carbon dioxide that escapes into the free atmosphere.

According to the method, a waste gas containing carbon dioxide, a hydrocarbon-containing gas is supplied to the waste gas. The hydrocarbon-containing gas reacts with the carbon dioxide of the waste gas, the reaction producing at least fractions of carbon monoxide (CO) and hydrogen (H₂) as reaction products. The waste gas containing the carbon monoxide/hydrogen mixture in suitable concentration is employed in a further combustion process. In this case it may be temporarily stored beforehand. The further combustion process can be, but does not necessarily have to be, part of the process in which the treated waste gas accumulates.

In an embodiment variant, the carbon monoxide/hydrogen mixture (referred to hereinbelow for simplicity as fuel gas) has a higher calorific value than the introduced hydrocarbon-containing gas (referred to hereinbelow as reforming gas). This means in turn that the reaction that takes place between the reforming gas and the carbon dioxide is an endothermic reaction, i.e. a reaction which draws heat from its environment.

Thus, a substantial proportion of the environmentally harmful carbon dioxide is removed from the waste gas by the method and it can be supplied in converted form as fuel gas to a further combustion process. In this process the thermal energy of the waste gas is therefore converted into chemical energy of the generated fuel gas.

It has proved beneficial to use methane, in particular in the form of natural gas, for the hydrocarbon-containing reforming gas. In this case a strongly endothermic reaction sets in for the recovery of the carbon dioxide, leading to the formation of carbon monoxide and hydrogen.

The method is advantageously employed in steelmaking, since waste gas containing a substantial amount of carbon dioxide at high temperatures often occurs in steel production. In particular the waste gas of a converter in steelmaking is suitable for being treated according to the method. A converter used in steelmaking serves to reduce the carbon content in molten iron.

In an embodiment variant, water, e.g., in the form of vapor, can also be added to the waste gases in addition to the reforming gas. Supplying additional water causes a change in the ratio of carbon monoxide to hydrogen, which is beneficial as fuel gas in a variety of applications.

Because the carbon content of the waste gas is not constant at every instant in the process workflow, it is beneficial to monitor the waste gas. The carbon dioxide fraction of the waste gas can be monitored in particular through the installation of a gas sensor and the introduction of the reforming gas can be controlled accordingly.

It can also be beneficial to provide a collar on the converter, the element already being present on many known systems and reducing the ingestion of inleaked air, i.e. unwanted ambient air, with the result that no additional reaction can take place between oxygen and the reforming gas.

Furthermore the fuel gas can be temporarily stored in an already present gas container. The fuel gas can moreover be utilized in a variety of further processes, in particular in the steel industry. It can be used for example for generating electricity in a power station or for process steam production (possibly in combination with electricity generation). The fuel gas can also be used for slab, billet and bloom preheating in elevator furnaces or pusher furnaces or in burners. This applies for example to ladle drying and heating, to heating stations or to distributors in continuous casting plants.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and advantages will become more apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings of which:

FIG. 1 is a schematic diagram showing the method in the treatment of waste gases from a converter,

FIG. 2 is a block diagram of the original treatment of a waste gas in the steel industry according to the related art, and

FIG. 3 is a block diagram of the process according to FIG. 2 with additional waste gas reforming.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.

In the following it is aimed to explain the waste gas treatment process with reference to the example of a converter according to FIG. 1 as used in steel production. The converter 4 serves to remove excess carbon from an iron melt. Toward that end oxygen is injected into the iron and the carbon contained in the iron melt is oxidized to form carbon dioxide. For this reason a waste gas 2 of the converter 4 contains a considerable proportion of carbon dioxide.

The carbon dioxide content of the converter waste gas is dependent on its operating parameters. The proportion of carbon dioxide in relation to carbon monoxide in the waste gas 2 fluctuates as a function of the supply of oxygen and operating temperature. The waste gas 2 flows into a waste gas channel and is checked there by a probe 3 in order to determine its proportion of carbon dioxide. If the carbon dioxide fraction is above a preset threshold value, reforming gas 5 is conducted into the waste gas channel 11 by way of a reforming gas supply 6. The reforming gas 6, for which natural gas containing a high proportion of methane can be used for example, reacts with the carbon dioxide of the waste gas at least partially in accordance with the following reaction equation (dry reforming 7, cf. FIG. 3).

CH₄+C0₂→2CO+2H₂ΔH=+250 kJ/mol

This reaction is endothermic, with 250 kJ per mole of thermal energy being drawn from the environment, i.e. the waste gas 2. In this way thermal energy is converted by the reaction and stored as chemical energy in the fuel gas 7 formed (CO+H₂, also called synthesis gas). Accordingly, thermal energy is therefore converted into chemical energy, since the fuel gas 9 resulting according to equation 1 has a higher calorific value than the originally used reforming gas (methane).

The individual calorific values of the educts and products are:

CH₄: 55.5 MJ/kg=888 MJ/kmol

CO: 10.1 MJ/kg=283 MJ/kmol

H₂: 143 MJ/kg=286 MJ/kmol

The calorific value of a mixture composed of 2 mole carbon monoxide and 2 mole H₂ is higher by the above-cited reaction enthalpy of 250 kJ/mol than the calorific value of one mole CH₄ (methane) from which the fuel gas 7 is produced. The increase in calorific value is therefore equal to 28% of the introduced calorific value of the methane (250 kJ/mol: 888 kJ/mol).

Depending on how the fuel gas 7 is used it can make sense to shift the CO:H₂ ratio in favor of the hydrogen. In this case water (e.g., in the form of steam) is optionally likewise introduced as well at the reforming gas supply 6. This enables an exothermic CO shift reaction to take place, according to which

H20+CO→C0₂+H₂ΔH=H-42 kJ/mol

the ratio of H₂ to CO is changed. Although this results in less waste heat being stored (since this is an exothermic reaction), a higher H₂ content is nonetheless achieved in the fuel gas 7, which is advantageous in certain combustion processes. That is the case in particular when the heat transfer in the combustion processes takes place by radiation and not by convection. As a result of the H₂ combustion a higher water content is produced in the waste gas, thus promoting the transfer of heat on account of water's wide radiation band.

In the present example of a converter, in particular in the case of a converter having a collar 13, the reforming treatment with the CO₂ of the waste gas 2 can be usefully used in two different process states of the reforming process. On the one hand this is the case in the so-called rampup and rampdown phases in which the waste gas is up until now not used because the C0₂ content of the waste gas is too high and the CO content too low. As a result of the described dry reforming, a usable fuel gas in accordance with equation 1 is obtained having sufficient calorific value, which gas will presently be described in relation to the further execution of the method and can be stored in a gas reservoir.

On the other hand the dry reforming process can also be applied in order to achieve a further increase in the calorific value of CO-rich gas that is already collected anyway according to the related art, if the fuel-rich gas is to be mixed with lean gases from other parts of the steelworks and the mixture does not possess sufficient calorific value for further combustion processes.

Using a collar 13 is beneficial in order to avoid ingestion of inleaked air which would lead to the combustion of the methane or natural gas, i.e. the reforming gas 7, instead of completing the described reforming according to equation 1. Furthermore, the high nitrogen content of the air would lead to the dilution of the converter gas and fuel gas.

After the reforming process, the waste gas is cooled down in a steam generator 8, with steam being generated therein which can be used in turn for generating electricity.

This is followed by a coarse dedusting 10 of the waste gas 2 which is conducted further into an evaporation cooler 12. The evaporation cooler 12 is necessary because the waste gas must not be hotter than 180° C. for a succeeding dry electrostatic precipitation 14 in which the remaining fine dust is removed from the waste gas 2 by filtering. After the fine dust has been separated out, the waste gas 2 is conducted via a blower 10 and either burned off by way of a flare stack 18 or, after further cooling in a gas cooler 20, supplied to a gas container 22.

The question of whether the combustible constituents in the waste gas 2 are burned off by way of a flare stack or whether the high-caloric waste gas comprising a CO/H₂ mixture is stored as fuel gas 9 in the gas container 22 is dependent on the carbon dioxide fraction of the waste gas 2. Given suitable control of the reforming process, by way of a sensor 3 for example, the described reforming of the waste gas 2 with the reforming gas methane results in the proportion of the CO/H₂ mixture in the waste gas after the filtering being so high that the major part of the waste gas or, as the case may be, of the carbon monoxide and the hydrogen can be stored in the gas container 22 and can be reused as fuel gas 9. By this measure the waste gas that is burned off in the flare stack 18 is reduced to a very small amount compared with the related art.

The difference between the related art waste gas treatment process for converter off-gases and the method described herein is shown schematically once again in FIGS. 2 and 3 with the aid of a block diagram.

On the extreme left is a process in which C0₂ waste gas is generated, illustrated here with reference to an example of a converter 4 in which waste gas 2 is produced. The carbon dioxide-containing waste gas 2 is cooled down in a steam generator 8, resulting in steam being produced for further use. In addition there now follows an evaporation cooler 12 in which waste heat Q1 is produced, of which no further use is made in this case. Next follows a dry electrostatic precipitator 14, downstream of which, depending on the carbon dioxide content of the waste gas 2, the latter is burned off by way of a flare stack 18 or stored in a gas container 22 for further use as fuel gas 9.

The method described here, as illustrated in FIG. 3, differs from the method according to the related art as shown in FIG. 2 in that a reforming process 7 in the form of dry reforming takes place between the converter 4 and the steam generator 8, wherein reforming gas 5 is supplied to the process by way of a reforming gas supply 6 and the waste gas 2 is treated as described in equation 1.

In addition to the described insertion of the dry reforming process 7, a further difference between the two methods is that the amount of heat Q2 drawn off at the evaporation cooler 12 is less than the amount of heat Q1 of the evaporation cooler 12 according to FIG. 2 and that the volume m₂ of the gas 2 that is burned off at the flare stack 18 is less than the volume m₁ that is burned off at the flare stack 18′ according to the related art.

Because the fuel gas mixture can be used again in suitable installations of the steelworks, the production phases in which, according to the related art, a reburning is performed in the flare stack 18, can be reduced or shortened. As a result thereof the energy content of combustible components of the waste gas can be used in combination with the formed fuel gas in an advantageous manner with waste heat being stored. As a result of the shorter operating times of the flare stack the gas collection time is increased and the carbon dioxide emissions of the converter plant or, as the case may be, of the flare stack are reduced.

A description has been provided with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in Superguide v. DIRECTV, 358 F3d 870, 69 USPQ2d 1865 (Fed. Cir. 2004). 

1-11. (canceled)
 12. A method for treating a carbon dioxide-containing waste gas, comprising: supplying a hydrocarbon-containing gas to the waste gas; converting at least some of the carbon dioxide of the waste gas in a reaction into a carbon monoxide and hydrogen mixture; and supplying the carbon monoxide and hydrogen mixture to a combustion process.
 13. The method as claimed in claim 12, wherein the reaction in said converting of the carbon dioxide in the waste gas is endothermic with the hydrocarbon-containing gas and the waste gas cooled down as a result of the reaction.
 14. The method as claimed in claim 13, wherein the hydrocarbon-containing gas includes methane.
 15. The method as claimed in claim 14, wherein the waste gas accumulates in a steelmaking process.
 16. The method as claimed in claim 15, wherein waste gas accumulates in a converter in which the carbon content in molten iron is reduced.
 17. The method as claimed in claim 16, further comprising adding water to the waste gas in addition to the hydrocarbon-containing gas.
 18. The method as claimed in claim 17, further comprising monitoring the carbon dioxide in the waste gas by a gas sensor; and regulating said supplying of the hydrocarbon-containing gas based on said monitoring of the carbon dioxide content of the waste gas.
 19. The method as claimed in claim 18, wherein a collar is provided on the converter to avoid ingestion of inleaked air.
 20. The method as claimed in claim 19, further comprising storing the carbon monoxide and hydrogen mixture temporarily in a gas container.
 21. The method as claimed in claim 20, wherein the carbon monoxide and hydrogen mixture is used as a fuel gas in the combustion process.
 22. The method as claimed in claim 21, wherein said supplying of the hydrocarbon-containing gas occurs during at least one of a rampup process and a rampdown process of the converter. 